Hitherto, various sound absorption materials are known. As the sound absorption material, there is known a material such as a foam resin with an open cell or an aggregate of fiber having air permeability and air passage resistance. For example, Patent Document 1 discloses an expanding body with an open cell structure.
Further, for example, pages 66 and 67 of Non-Patent Document 1 disclose a sound absorbency performance of a porous sound absorption material and a porous sound absorption structure as below. Furthermore, “FIG. 4.2.1(a)” and “FIG. 4.2.1(b)” correspond to FIGS. 6(a) and 6(b) of the application.
As one of material characteristics of the “porous sound absorption material”, air permeability may be exemplified since a plurality of microscopic gaps or open cells illustrated in FIG. 4.2.1(a) exist in the material. When these constitute a sound absorption mechanism, sound energy is mainly converted into thermal energy due to the viscous friction while the sound enters the porous sound absorption material and penetrates the microscopic gaps or cells, and hence the sound energy is attenuated. Furthermore, even in the same expanding agent, the material having closed cells illustrated in FIG. 4.2.1(b) is a porous material, but is not included in the porous sound absorption material.
Further, in order to adjust a frequency with a maximal sound absorption coefficient or to obtain a protection from an exhaust gas or an UV ray, there is known a technique for providing a member without air permeability on a surface of a material with air permeability. For example, Patent Document 2 discloses a technique in which a resin sheet is provided on both surfaces of a resin expanded sheet with only an open cell layer. Further, for example, Patent Document 3 discloses a technique in which a flat plate without air permeability and a gel layer are laminated with an air chamber layer (aggregate) having air permeability interposed therebetween. Further, for example, Patent Document 4 discloses a technique in which a non-woven fabric and a crystalline resin film are laminated on both surfaces of an open cell expanded layer.
Further, there is known a technique in which a plurality of air permeable materials having different sound absorbing characteristics are laminated so as to further increase a sound absorption coefficient at a specific frequency. For example, Patent Document 5 discloses a technique in which a first layer without air permeability, a second layer with air permeability, and a third layer with open cells are laminated.
Furthermore, Patent Document 6 discloses a technique in which a hole is formed in a metal foil or a metal thin sheet.